1.
A = ( 1 0 0 1 / 2 0 1 / 2 ) , b = ( 2 4 6 ) A=\begin{pmatrix}
1 & 0 \\
0 & 1/\sqrt 2\\
0&1/\sqrt 2
\end{pmatrix},b=\begin{pmatrix}
2 \\
4\\
6
\end{pmatrix} A = ⎝ ⎛ 1 0 0 0 1/ 2 1/ 2 ⎠ ⎞ , b = ⎝ ⎛ 2 4 6 ⎠ ⎞
Then u is the orthogonal projection of b onto the subspace spanned by the column of A and it is given by the formula:
u = A ( A T A ) − 1 A T b u=A(A^TA)^{-1}A^Tb u = A ( A T A ) − 1 A T b
then:
A T b = ( 1 0 0 0 1 / 2 1 / 2 ) ( 2 4 6 ) = ( 2 10 / 2 ) A^Tb=\begin{pmatrix}
1 & 0&0 \\
0 & 1/\sqrt 2& 1/\sqrt 2\\
\end{pmatrix}\begin{pmatrix}
2 \\
4\\
6
\end{pmatrix}=\begin{pmatrix}
2 \\
10/\sqrt 2
\end{pmatrix} A T b = ( 1 0 0 1/ 2 0 1/ 2 ) ⎝ ⎛ 2 4 6 ⎠ ⎞ = ( 2 10/ 2 )
A T A = ( 1 0 0 0 1 / 2 1 / 2 ) ( 1 0 0 1 / 2 0 1 / 2 ) = ( 1 0 0 1 ) A^TA=\begin{pmatrix}
1 & 0&0 \\
0 & 1/\sqrt 2& 1/\sqrt 2\\
\end{pmatrix}\begin{pmatrix}
1 & 0 \\
0 & 1/\sqrt 2\\
0&1/\sqrt 2
\end{pmatrix}=\begin{pmatrix}
1 & 0 \\
0 & 1
\end{pmatrix} A T A = ( 1 0 0 1/ 2 0 1/ 2 ) ⎝ ⎛ 1 0 0 0 1/ 2 1/ 2 ⎠ ⎞ = ( 1 0 0 1 )
( A T A ) − 1 = ( 1 0 0 1 ) (A^TA)^{-1}=\begin{pmatrix}
1 & 0 \\
0 & 1
\end{pmatrix} ( A T A ) − 1 = ( 1 0 0 1 )
( A T A ) − 1 A T b = ( 1 0 0 1 ) ( 2 10 / 2 ) = ( 2 10 / 2 ) (A^TA)^{-1}A^Tb=\begin{pmatrix}
1 & 0 \\
0 & 1
\end{pmatrix}\begin{pmatrix}
2 \\
10/\sqrt 2
\end{pmatrix}=\begin{pmatrix}
2 \\
10/\sqrt 2
\end{pmatrix} ( A T A ) − 1 A T b = ( 1 0 0 1 ) ( 2 10/ 2 ) = ( 2 10/ 2 )
u = ( 1 0 0 1 / 2 0 1 / 2 ) ( 2 10 / 2 ) = ( 2 5 5 ) u=\begin{pmatrix}
1 & 0 \\
0 & 1/\sqrt 2\\
0&1/\sqrt 2
\end{pmatrix}\begin{pmatrix}
2 \\
10/\sqrt 2
\end{pmatrix}=\begin{pmatrix}
2 \\
5\\
5
\end{pmatrix} u = ⎝ ⎛ 1 0 0 0 1/ 2 1/ 2 ⎠ ⎞ ( 2 10/ 2 ) = ⎝ ⎛ 2 5 5 ⎠ ⎞
2.
x 2 − 4 ≥ 0 ⟹ ∣ x ∣ ≥ 2 x^2-4\ge 0 \implies |x|\ge 2 x 2 − 4 ≥ 0 ⟹ ∣ x ∣ ≥ 2
then image of S:
f ( x ) ≤ 2 ( − 2 ) − 1 = − 5 f(x)\le 2(-2)-1=-5 f ( x ) ≤ 2 ( − 2 ) − 1 = − 5 and
f ( x ) ≥ 2 ⋅ 2 − 1 = 3 f(x)\ge 2\cdot 2-1=3 f ( x ) ≥ 2 ⋅ 2 − 1 = 3
3.
for ker(T):
U + V = 0 U+V=0 U + V = 0
U + 2 U = 0 U+2U=0 U + 2 U = 0
so,
k e r ( T ) = ( 0 , 0 ) ker(T)=(0,0) k er ( T ) = ( 0 , 0 )
4.
by the rank-nullity theorem:
6 = d i m ( K e r ( T ) ) + d i m ( I m ( T ) ) 6=dim(Ker(T))+dim(Im(T)) 6 = d im ( Ker ( T )) + d im ( I m ( T ))
So, dim range T= 6-dim(Ker(T))=6-2=4
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